Nuclear generators naturally involve nuclear cores that produce decay thermal energy after shut down. Generally, among several factors, the amount of decay thermal energy produced after shutdown is proportional to the fuel power generation history and power density characterizing the nuclear core. To avoid overheating of the nuclear fuel in any location of the core, decay heat energy must be transferred from the core using redundant heat transfer mechanisms generally supported by systems external to the vessel and structures designed to contain the core. These redundant cooling systems comprise complex networks of piping thermal-hydraulically coupling the core to heat exchangers located outside of the vessel containing the core and dedicated to transfer thermal energy from the core to the environment (i.e. an ultimate heat sink). Coolant through these heat exchangers may actively circulate using electrically driven re-circulators (i.e. pumps, blowers) and redundancies are represented using multiple heat exchangers regulated by valves dedicated to route or re-route coolant through relatively complex piping networks. Alternatively, coolant may passively circulate through similarly complex piping networks, thermal-hydraulically coupling the core to extra-core heat exchangers, by gravity-driven natural circulation mechanisms based on the fact that coolant density changes when heated or cooled. Modern nuclear reactors rely on redundant core decay heat removal systems that may be operated passively, actively or a combination of both.
To remove decay thermal energy, reactor designs adopting “active” safety features extensively rely on electric power for the core to be maintained at safe temperatures after shutdown. To ensure safe operation and decay thermal energy removal at all times, these designs require electric power provided by connection to a minimum of two off-site power grids, and emergency electric power produced by dedicated redundant on-site emergency diesel generators (EDGs).
Some types of passive safety features, on the other hand, solely rely on gravity and large inventory of water generally stored in tanks or water structures positioned at relatively high elevations with respect to the core. Elevation differential between the core and the coolant storage structures is required for the coolant to undergo natural circulation siphoning, and effectively remove decay thermal energy from the core. For passive safety features based on stored coolant, the ability to adequately provide long-term decay heat removal is highly dependent on the coolant inventory and the effectiveness of the gravity-driven core-cooling mechanism under various environmental temperature and humidity conditions. Generally, as environmental temperature increases, the ability to passively generate convective core-cooling becomes gradually impaired. As a result passive decay heat removal based on stored coolant inventories is best suitable for nuclear generators operating in mild climates.
As passive and active safety systems generally develop externally to the vessel housing the core, the result is a complex system of redundant piping, valves, heat exchangers, as well as pumps/blowers and ancillary power and control cabling networks (i.e. required to provide motive-electric power and control for active systems). The complex system of piping and thermal-hydraulic (i.e. heat exchangers) and electric equipment (i.e. pumps) dedicated to remove thermal energy from the core is generally defined as balance of plant. The balance of plant of most nuclear generators, large and small, induces substantially large plant foot-prints, imposes limitations on the sites at which the nuclear generators can be deployed, and significantly increases the capital cost characterizing nuclear generator installations.
Nuclear cores of commercially operating reactors are generally cooled by water and loaded with nuclear fuel elements cladded with materials that oxidize in the presence of high temperature water/steam. As a core may experience overheating due, for example, to loss of coolant, or failure of the active or passive core decay heat removal systems, chemical reactions between cladding materials and water/steam result in the production of hydrogen. Hydrogen then accumulates and eventually self-ignites, thereby posing severe safety challenges. As a result, nuclear power plants are equipped with redundant hydrogen management equipment to, for example, execute controlled ignitions and prevent accumulation of large hydrogen amounts. However, this additional safety feature further adds complexity, increases operating cost and may not be as manageable as demonstrated by several nuclear accidents as, for example, the accident that occurred at the Fukushima Daiichi nuclear station in Japan. The level of redundancies employed to ensure active, passive, or a combination of both safety systems, execute they safety functions are generally the result of probabilistic risk assessments based on postulated design basis accident scenarios. Not all possible accident scenarios are contemplated as the probability for the occurrence of beyond design basis accident scenarios is very low. Unfortunately, despite redundancies and multiple engineered barriers to the escape of radioactivity from the core to the environment, core meltdown, hydrogen explosions, containment breach and large radioactive fall out have occurred even for nuclear generating stations compliant with the most up to date regulatory guidance for safe operation (i.e. Fukushima Daiichi power station), thus demonstrating that catastrophic accidents, as those triggered by beyond design basis accident scenarios, have an unacceptable safety and economic impact even though their probability of occurrence is very low. Beyond design basis accident scenarios may be represented by extreme seismic, tsunami, weather related, terrorist/hostile events.
Small modular reactor designs are characterized by smaller, modular and more easily transportable components when compared to large modern reactor designs. However, these components, or modules, cannot operate without first being thermal-hydraulically (and electrically) coupled at the site of deployment. Coupling of these modular components occurs by interconnection with complex networks of piping, valves, passive and/or active core cooling systems (balance of plant), configured outside of the vessel comprising the core. As a result deployment, and installation of an electric station based on small modular reactor designs, requires several months for site preparation, installation of balance of plant equipment, and coupling of all auxiliaries regardless of the size of the small modular reactor. In fact, once small modular reactor systems are coupled, the total small modular reactor-based electric station footprint and emergency evacuation zone remain still substantial, even for small modular reactor designs producing modest or very low power ratings. Once assembled, small modular reactor designs cannot be transported or retrieved and therefore cannot be readily deployed nor they can be retrieved from a site without undergoing disassembly of modular components and several months dedicated to dismantling the balance of plant, with generally lengthy decommissioning procedures for the removal of several separate and potentially radioactive small modular reactor components.